Engineers at the Georgia Institute of Technology are adapting aerodynamic control techniques originally developed for aircraft to help the boxy truck trailers move along the highway as if they were streamlined. The technique, called a circulation control system, involves blowing compressed air from a series of slots on the trailers. Air exiting from the slots improves the way that wind flows over the trailers as they move along the highway.

The system is expected to reduce aerodynamic drag by at least 35 percent, which would cut fuel consumption for tractor-trailers by approximately 12 percent. If used on the entire U.S. heavy truck fleet, the technology could save an estimated 1.2 billion gallons of fuel a year. Beyond the fuel savings, however, circulation control could also improve directional control for the trucks, increase traction, and augment braking. The system could also create lift on the trailers, effectively reducing their weight and cutting rolling resistance on tires, reducing tire wear and further increasing fuel economy.

The system works by blowing compressed air from slots located on different parts of a trailer. Air blown over curved surfaces on the top smoothes the wind flow there, decreasing drag and making the entire trailer act like a wing to lift as much as 15 percent of the weight off the tires. Blowing air from slots on the bottom of a trailer would have the opposite effect, multiplying downward force on the tires to improve traction and braking when needed.

Blowing slots on each side of a trailer would counter crosswinds, giving the driver a way to fight the effects of sway. By selecting the right slot combination and blowing rates, the system would increase drag to augment braking, particularly helpful for trucks going down steep hills.

Controlled by internal fast-acting valves, the system would respond quickly to driver actions. Using computerized controls, the driver wouldn't even have to think about how it works. Pressing the brakes or accelerator, or moving the steering wheel, would cause the proper blowing slots to open and assist the conventional controls. Compressed air for the system could come from the exhaust gases, the turbocharger on the truck engine, storage tanks, or an electric compressor in the trailer.

Circulation control systems were developed and tested on fixed-wing and rotary-wing aircraft in the 1970s and 1980s as a simplified means of greatly increasing lift, improving control, and reducing take-off and landing distances. During the 1990s Georgia Tech engineers applied the technique to automobiles and demonstrated significant savings in drag and energy use.

The potential value of the system has been demonstrated in wind-tunnel testing on truck models and testing on full-sized truck trailers. If it proves successful, the system could quickly become part of new trailers and could be sold as aftermarket kits for existing trailers. The research is being sponsored by the U.S. Department of Energy.